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A study on corrosion behavior of austenitic stainless steel in liquid metals at high temperature
Journal of Nuclear Materials 422 (2012) 92–102
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A study on corrosion behavior of austenitic stainless steel in liquid metals at high temperature Sang Hun Shin, Jong Jin Kim, Ju Ang Jung, Kyoung Joon Choi, In Cheol Bang, Ji Hyun Kim ⇑ Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), 100 Banyeon-ri, Eonyang-eup, Ulju-gun, Ulsan 689 798, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 July 2011 Accepted 10 December 2011 Available online 28 December 2011
a b s t r a c t The purpose of this study is to investigate the interaction between austenitic stainless steel, AISI 316L, and gallium liquid metal at a high temperature, for the potential application to advanced fast reactor coolants. Test specimens of AISI 316L were exposed to static gallium at 500 °C for up to 700 h in two different cover-gas conditions, including air and vacuum. Similar experimental tests were conducted in gallium alloy liquid metal environments, including Ga–14Sn–6Zn and Ga–8Sn–6Zn, in order to study the effect of addition of alloying elements. The results have shown that the weight change and metal loss of specimens were generally reduced in Ga–14Sn–6Zn and Ga–8Sn–6Zn compared to those in pure gallium at a high temperature. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For recycling spent fuels from an operating nuclear power plant, the liquid metal fast breeder reactor (LMFBR) is one of the most promising candidates among Gen-IV nuclear energy systems. Among the various liquid metals used as a primary coolant, sodium is a spotlighted coolant for designing fast breeder reactors. However, sodium has a disadvantage in that it has high activity with water and air. This factor drives the search for alternatives. In this aspect, heavy liquid metals, including pure lead and lead–bismuth eutectic (LBE) alloy, have been extensively studied for fast reactor application. However, there have also been issues such as corrosion with structural alloys at a high temperature, and Po-generation in LBE cooled nuclear systems. This study focused on another liquid metal, gallium, as a potential coolant for the next generation nuclear reactor system. The element of gallium possesses unique properties as follows: It is very stable in air or water, has a very low melting point, and a very high boiling point. It melts at 29 °C and the boiling point is as high as 2204 °C, which gives a designer a larger margin before coolant boiling if severe accidents happen. Being a liquid metal, the heat transfer characteristics would be good, though not as good as some other liquid metals, such as sodium. There is no issue with gallium in terms of chemical activity, or radioactivity increase by irradiation in a nuclear reactor environment. Because of these advantages in using liquid gallium, there have been several studies in the past for applying liquid gallium to nuclear systems [1–3]. The absorption cross section of gallium is rather high at 2.2 barns per atom ⇑ Corresponding author. Tel.: +82 52 217 2913. E-mail address: [email protected] (J.H. Kim). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.12.007
[4], which is a drawback, but it can be managed to a certain degree, because this could be reduced by proper alloying with elements with a lower absorption cross section. When gallium is alloyed with tin and zinc which have a cross section of 0.63 and 1.10 barns per atom, respectively, in a certain ratio, the melting point would be 21 °C. The thermal conductivity of pure gallium and alloying elements with gallium, tin and zinc at 30 °C is 29 W/m K, 66 W/ m K, and 116 W/m K, respectively [4,5]. Tin and zinc are the materials which have both the ternary eutectic and a lower absorption cross section than gallium [3]. When viewed for compatibility as a coolant, which is the main interest of this study, gallium has a relatively high affinity for many metals and alloys, especially steels. Therefore, there has been a relatively high corrosion of these structural metals and alloys, due to the dissolution of their various constituents by the liquid gallium [3,6–8]. Since gallium had some promise as a reactor coolant, owing to its unique properties as described above, the studies on determining the effects of alloying on the melting point and cross section, and on understanding the corrosion of possible structural materials by gallium, were merited. In this study, a series of corrosion tests of structural materials were performed in static gallium at 500 °C for up to 700 h, in both air and vacuum cover-gas conditions, and analyzed to understand the general corrosion behaviors of structural materials in liquid gallium at a high temperature. Also, the effect of adding alloying elements, including tin and zinc, to gallium were investigated. 2. Experimental In this study, austenitic stainless steel, AISI 316L, one of the most popular structural materials in the nuclear industry, was used
S.H. Shin et al. / Journal of Nuclear Materials 422 (2012) 92–102 Table 1 Chemical composition of SS 316L used in this study. Element
Fe
Cr
Ni
Mo
Mn
Si
Other
wt.%
Bal.
16.43
10.05
2.02
1.02
0.66
–
Fig. 1. Optical micrograph of austenitic 316L stainless steel (magnification: 200).
for the various corrosion tests. Table 1 shows the chemical composition of the AISI 316L used in this study. A specimen was etched with 100 mL of ethanol, 100 mL of HCl and 5 g of CuCl2 for 30 s at room temperature. Fig. 1 shows the typical microstructure of the material, obtained by optical microscope after etching. All specimens for the corrosion test had dimensions of 30 mm in length, 10 mm in width and 3 mm in thickness, and were cut from an AISI 316L plate by a high pressure water jet process to avoid thermal stress. Prior to exposure to gallium, each specimen was mechanically polished by SiC papers, diamond suspension (6 lm, 3 lm, and 1 lm), and finally alumina paste down to 0.04 lm, then ultrasonically cleaned with demineralized water for 30 min. Each specimen was placed in an alumina crucible filled with liquid gallium heated to about 40 °C to avoid precipitate from the container. Tests were conducted in two different cover-gas condi-
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tions, including air and vacuum. After introduction of the specimen, the crucible was placed in a furnace with air for the test in air cover-gas conditions. For the test at high vacuum cover-gas conditions, the crucible was placed in a furnace with a diffusion pump operating up to 5 106 torr. All the specimens were exposed to gallium at 500 ± 5 °C for times between 17 and 700 h. After exposure to gallium and gallium alloys, all specimens were first rinsed in 50 °C water to remove gallium residue from the surface, and then ultrasonically cleaned in demineralized water to ensure complete removal of gallium. After drying, all specimens were weighed, and their dimensions were measured. Subsequently, the specimens were sectioned perpendicular to their length and mechanically polished to examine the cross sections by optical and scanning electron microscopy (SEM) composition of the phases was determined by using energy dispersive X-ray spectrometry (EDS), electron probe X-ray microanalysis (EPMA), and time of flight secondary ion mass spectrometry (TOF-SIMS). Elemental concentration profiles were obtained in the reaction zone to determine the composition of the phases. To quantify the corrosion behavior, the weight change, metal loss, and thickness of the reaction layer were measured in each post-test specimen. For these measurements, both sides of the rectangular cross section were considered.
3. Results Fig. 2 shows the typical optical micrographs of the cross-section around the edge region of post-test specimens. As shown in Fig. 2, specimens exposed to gallium for 17 h still retain their rectangular shape, but those exposed to gallium for 700 h lost the corners of the rectangles. The thickness of the reaction layer formed by the interaction between steel and gallium was not uniform, especially at the corners of the rectangle, but those on the side flat surfaces were observed to be uniform in each specimen, because the reaction layer dominantly formed to width direction with time. Also, with the increase of the exposure time, the volume and weight of the specimens decreased, but the thickness of the reaction layer increased in general. There is no clear evidence for a preferential
Fig. 2. Optical images of specimens after exposure to liquid gallium at 500 °C for (a) 17 h, (b) 700 h in air cover gas condition, (c) 17 h, and (d) 700 h in vacuum cover-gas condition, respectively.
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Fig. 3. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 17 h at 500 °C in air cover-gas condition.
attack along the grain boundary which is commonly observed in some liquid metal environments at a high temperature [4,8–10]. 3.1. Corrosion tests in Ga liquid metal All the specimens were found to form reaction layers on the outer. Figs. 3 and 4 show the SEM images and maps of cross sectional chemical composition around the reaction layers, formed on the specimens exposed to liquid gallium with air cover-gas at 500 °C for 17 and 700 h, respectively. The SEM images (5000 magnification) in Fig. 5 show the clear interface between the base metal and the compound region. Referring to the grain structure, as shown in Fig. 1, it is evident that there is no preferential attack along grain boundaries, along with the quantitative elemental profiles around the interface between the base metals and the reaction layers by the EPMA of the post-test samples. Comparing the SEM images in Figs. 3a and 4a, the thickness (250 lm) of the reaction layer in the specimens exposed to liquid gallium for 17 h is much thinner than that (850 lm) of the specimens exposed to liquid gallium for 700 h. It is also noted that there exists a fissure for longer exposure tests as shown in Fig. 4a. The results of the maps of the cross sectional chemical composition show that the reaction layers are mainly composed of gallium from the liquid metals bath, and iron, chromium, and nickel from the test specimens. The solubility data of the specimen constituents in gallium has not been widely studied. The main constituents of the specimen are iron, chromium and nickel. At
500 °C, the solubility of iron, chromium and nickel in gallium is 0.104 at.%, 0.049 at.% and 1.34 at.%, respectively. Nickel is highly soluble in gallium compared to other constituents at the temperature. Oxygen observed on the outermost surface of the reaction layer is considered to form a thin layer of oxide with gallium in the beginning stage of the reaction. As seen from elemental profiles by EPMA (Fig. 5) along the interface between the base metals and the reaction layers, transition of each chemical element is much sharper in the specimens subjected to longer exposure, than those exposed for shorter lengths of time. Figs. 6–8 show the SEM images, maps of cross sectional chemical composition, and the quantitative EPMA profiles, around the reaction layers formed on the specimens exposed to liquid gallium with vacuum cover-gas at 500 °C for 17 and 700 h, respectively. Compared to the results in air cover-gas conditions, the thickness of the reaction layer in the vacuum cover-gas conditions is slightly reduced (200 lm) after 17 h exposure to gallium. However, after 700 h exposure it showed a significant gain (1000 lm), as seen from Figs. 6a and 7a. The EPMA maps of the cross sectional chemical composition and elemental profiles around the reaction layer did not show a significant difference between the two different cover-gas conditions. 3.2. Corrosion tests in Ga–14Sn–6Zn liquid metal As same as performed in the specimens exposed in pure Ga liquid metal, maps of cross sectional chemical composition, and
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Fig. 4. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 700 h at 500 °C in air cover-gas condition.
the quantitative elemental profiles by EPMA are acquired from the reaction layers formed on the specimens exposed to Ga–14Sn–6Zn liquid metal with air cover-gas at 500 °C for 17 and 700 h, respectively. Detail images and elemental profiles are given in a recent study of Shin [11]. Similar to the results in the pure liquid gallium test, there is no preferential attack along grain boundaries, but a significant change is seen in the thickness of the reaction layer with increased exposure time in Ga–14Sn–6Zn liquid metal. Compared to the thickness of the reaction layer formed in the pure gallium test, a relatively thinner layer is observed to develop. The reaction layers are seen to reach to thicknesses of about 200 lm after 17 h, and 750 lm after 700 h in air cover-gas conditions, while those in vacuum cover-gas conditions are observed to grow to about 200 lm after 17 h and 800 lm after 700 h. In the literature, the solubility of iron in gallium at 500 °C is 0.104 at.%, while iron in zinc is 0.128 at.%, respectively [12–13]. The solubility of iron in tin at 500 °C is not available, but it shows almost same value of that in zinc at 800 °C [14]. In the case of chromium, the solubility limit in gallium, tin, and zinc at 500 °C is 0.049 at.%, 0.096 at.% and 1.48 at.%, respectively [12,15–16]. The solubility of nickel in gallium is 1.34 at.%, but there is little previous study of nickel’s solubility in tin and zinc. In AISI 316L stainless steel which is mainly composed of 69 wt.% of iron and 16.4 wt.% of chromium in this study, therefore, it would be expected that the amount of SS316L dissolution in tin, and probably in zinc, are larger than that in gallium at this
temperature. However, these solubility limits for both iron and chromium in each liquid metal does not agree with the observation in this study. The test results show that all specimens exposed to pure gallium have higher metal loss and larger reaction layer thickness than those exposed to gallium alloys at the temperature. The possible reason for this behavior is that the corrosion process is mainly controlled by diffusion, not by solubility limit, which will be discussed in detail in Section 4. A feature of fissure is also observed in longer exposure tests, irrespective of cover-gas conditions. In the chemical composition around the reaction layer, specimens exposed to Ga–14Sn–6Zn showed a slight difference compared to those exposed to pure gallium. While the reaction layers are mainly composed of elemental species from both the liquid metal bath and the base metal, similar to the specimens exposed to pure gallium liquid metal, the major difference is that small amounts of zinc (1–2 wt.%) and tin (0.1– 0.2 wt.%) were detected in the reaction layer, as seen from the cross sectional chemical composition maps, as well as in the quantitative elemental profiles by EPMA. 3.3. Corrosion tests in Ga–8Sn–6Zn liquid metal As with the results of the tests in Ga–14Sn–6Zn liquid metal, a significant gain in thickness of the reaction layer was observed with increased exposure time in Ga–8Sn–6Zn liquid metal. The reaction layer was also found to contain a small amount of zinc
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Compound
Base metal
Base metal
Compound
10 µm
(b)
80 70 60 50 40 30 20 10 0 -10
Fe Cr O Ga Ni
wt (%)
wt (%)
(a) 80 70 60 50 40 30 20 10
Fe Cr O Ga Ni
0 -10 -20
-10
0
10
20
Depth (µm)
(c)
-20
-10
0
10
20
Depth (um)
(d)
Fig. 5. SEM images of specimens exposed for (a) 17 h, (b) 700 h, and elemental profiles by EPMA for (c) 17, and (d) 700 h to liquid gallium at 500 °C in air cover-gas condition. Red (base metal) and blue (compound region) spots in SEM images (above) indicate the analyzed positions for EPMA, and quantitative elemental profiles of Fe, Cr, O, Ga, and Ni are obtained by EPMA (below). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(1–2 wt.%) from the quantitative elemental profiles by the EPMA measurements. In this case, there is no preferential attack along grain boundaries, as same as observed in pure gallium and Ga– 14Sn–6Zn. 3.4. Measurement of weight change, metal loss and reaction layer thickness The weight change, metal loss, and reaction layer of specimens after exposure to gallium and gallium alloys at high temperature were measured with post-test specimens, and the summarized results as a function of exposure time are plotted in Fig. 9 for air cover-gas tests, and in Fig. 10 for vacuum cover-gas tests, respectively. Generally, the weight of specimens decreased, but metal loss and the thickness of the reaction layer increased with the increase of the exposure time. In the air cover-gas conditions, the weight of the specimens was observed to change in negative proportion to the exposure time in liquid pure gallium, but the liquid gallium alloys showed a different pattern of change; a parabolic decrease with increase of time. After 700 h exposure, the weight change (804 mg/cm2) of specimens in pure gallium is about 2–3 times higher than that in Ga–14Sn–6Zn (459 mg/cm2) or Ga–8Sn–6Zn (324 mg/cm2) as shown in Fig. 9a. Also, the metal loss and reaction layer thickness increase with exposure time, which is commonly observed in the oxidation of metals and alloys at high temperatures. As shown in Fig. 9b, c and Fig. 10b, c, even during the shorter exposure time (17 h), the significant changes in both metal loss and reaction layer thickness were observed, which affects the weight change consequently as
they are directly related. However, the time dependence of the data squared shows that one obtains straight lines only after a transient period of about 17 h in most cases. In other words, the variation of the metal loss squared and reaction layer thickness squared versus exposure time cannot be described by a straight line for durations less than 17 h. This indicates that different kinetic regimes are involved during the process. In the initial stage (time less than 17 h), a faster growth appears with nonparabolic kinetics. After this transient period, the plot of the variation of the metal loss and layer thickness squared versus time can be represented by a straight line, indicating a parabolic behavior for the process. The change from nonparabolic to parabolic behavior can also be distinguished by the morphological change of the reaction layer. In the early stage, the reaction layer started to grow forming a cruciform pattern with sharp corners as shown in Fig. 2a and c whereas rounded edges appeared for longer times as shown in Fig. 2b and d. This type of transitional kinetic behavior of 316L steel has been reported for SS316L exposed to pure gallium with vacuum cover-gas condition in the previous study [8]. In the current study, the similar kinetics change for the weight loss and reaction layer thickness are observed in SS316L specimens exposed to gallium alloys with different cover-gas conditions. Compared to the data in gallium alloy conditions, specimens tested in pure gallium showed larger metal loss and layer thickness in all exposure times, except in metal loss after 700 h in Ga–8Sn– 6Zn. The wetted residue of liquid metal in post-test specimens could cause the scattering in measurement. As shown in Fig. 10a, in vacuum cover-gas conditions, the result showed that the weight change after 700 h exposure is 659 mg/cm2 in pure gallium, but 239 mg/ cm2 and 332 mg/cm2 in Ga–14Sn–6Zn and Ga–8Sn–6Zn,
S.H. Shin et al. / Journal of Nuclear Materials 422 (2012) 92–102
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Fig. 6. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 17 h at 500 °C in vacuum covergas condition.
respectively. Compared to the results tested in air cover gas conditions, specimens tested in vacuum cover-gas conditions showed less weight change in all liquid metal conditions. The metal loss and layer thickness showed the parabolic increase with exposure time, the same as those in air cover-gas conditions. Specimens tested in pure gallium conditions showed larger metal loss and layer thickness in all exposure time than those tested in gallium alloy conditions, as shown in Fig. 10b and c. There is no clear trend in the relationship between metal loss and layer thickness, and the cover-gas conditions for this test, when comparing the data in the air and vacuum cover-gas conditions.
4. Discussion From the characterization of post-test specimens by SEM, EDS and EPMA in this study, the interaction between austenitic stainless steel, AISI 316L, and liquid metals including gallium and gallium alloys at a high temperature resulted in the formation of a reaction layer consisting of the major constituents of the test specimens including iron, chromium, and nickel, as well as liquid metals including gallium, tin and zinc. From the literature data [17–19], thermodynamic equilibrium phases at isothermal (500 °C) binary system including Fe–Ga, Cr– Ga, Ni–Ga are identified in case of pure gallium environment,
and the summary is given in Table 2. The results of EPMA quantitative analysis on the reaction layer as shown in Section 3 indicates the observance of 65–75 wt.% of gallium, 20–25 wt.% of iron, 3–5 wt.% of chromium and 1–3 wt.% of nickel. These compositions allow gallium rich phases, including FeGa3, CrGa4 and Ni2Ga3 at 500 °C in the reaction layer, and the thermodynamic data [17–19] prove that these phases are likely to be formed due to the lowest formation of energy. TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) analysis was also performed in this study and the results indicated the presence of FeGa3, CrGa4 and Ni2Ga3 phases in the reaction layer of the specimen exposed to pure gallium, as shown in Fig. 11. In liquid gallium alloys (Ga–14Sn–6Zn and Ga–8Sn–6Zn), tin and zinc are composed in the liquid bath with a total amount of 20 wt.% and 14 wt.%, respectively. Specimens exposed to these environments also develop reaction layers with different compositions compared to specimens exposed to pure gallium. The authors expected that the reaction layer would be composed of a similar amount of tin and zinc. In liquid gallium–tin–zinc alloy tests, however, small amounts of zinc (1–2 wt.%) and tin (0.1–0.2 wt.%) were detected in the reaction layers by EPMA. TOF-SIMS analysis was also conducted to carefully investigate the phases on the reaction layer. All detected binary alloys in the reaction layer formed in gallium alloy environments, are FeGa3, Ni2Ga3, CrGa4, Sn, Zn, FeSn, FeZn10 and FeZn13 as shown in Fig. 12. From Fig. 12, the strong
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Fig. 7. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 700 h at 500 °C in vacuum covergas condition.
peaks of Zn, Sn and FeSn are observed, but the signals of FeZn10 and FeZn13 are weakly detected. Thermodynamically, FeSn and Fe3Zn10 are the most stable phases in the range of chemical composition of iron (70%), tin (8–14%), and zinc (6%) in the range of the test temperature in this study [20]. TOF-SIMS analysis shows the presence of FeSn phase, but not the Fe3Zn10 phase. By combining the results described above, it is considered that the EPMA signals of zinc, not tin, in the reaction layer, comes from the Fe3Zn10 phase formed during the exposure to Ga–Sn–Zn alloy liquid metals. As summarized in the Section 3, the test results showed that all specimens exposed to pure gallium have higher metal loss and larger reaction layer thickness than those exposed to gallium alloys. The solubility limits of metallic constituents in liquid gallium and alloys, by themselves, could not explain the observed corrosion behavior in gallium alloy environments in this study, because the solubilities of iron and chromium in tin, and probably in zinc, are higher than those in gallium at the temperature as described previously. The possible reason for this behavior is that the corrosion process in this study is mainly controlled by diffusion after a nonparabolic stage for short exposure times. Using the self-diffusion coefficients in pure liquid metals given in the literature [21–23], the diffusivities of the metallic constituents can be estimated by the expression of Thomaes and Itterbeek [24]. For the estimation of the diffusivities of the metallic constituents, 22.81, 5,82 and 2.98 are used as the self-diffusion coefficients (DAA, 105 cm2/s) for gallium, tin and zinc at 500 °C, respectively
[21–23], and the estimated results are summarized in Table 3. The comparison with literature data of diffusion coefficient of iron [25] and nickel [26,27] in zinc bath shows that the estimation is well in agreement having the same order of magnitude at 500 °C. From the estimated diffusivities of the metallic constituents in liquid gallium and alloys, it is seen that all the diffusivities of metallic constituents are higher in liquid gallium than in liquid tin and/ or zinc when dissolved metallic constituents diffuse out into the liquid bath. Therefore, it is generally expected that the diffusivities of metallic constituents are higher in liquid gallium than in liquid gallium alloys, and it consequently results in higher metal loss and larger reaction layer thickness in liquid gallium than those in liquid gallium alloys as observed in this study. The results in this study also indicate that there is no clear trend in the relationship between the metal loss and reaction layer thickness, and the cover-gas conditions, when comparing the data in the air and vacuum cover-gas conditions. In the air cover-gas environment, oxygen could be the key impurity that may affect the liquid metal corrosion. Commonly, for a light liquid metal (sodium and sodium–potassium alloy), the corrosion of iron based steels increases with the oxygen concentration in the liquid metal, while for a heavy liquid metal (lead and lead–bismuth alloy), the presence of oxygen may result in protective oxide layers that inhibit the corrosion [28]. According to Zhang et al. [28], the solubility of metallic constituent in liquid metal can increase with the dissolved oxygen concentration, and there exists an upper and lower
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Compound
Base metal
Base metal
Fe Cr O Ga Ni
0
10
20
Depth (µm)
(c)
wt.(%)
wt.(%)
80 70 60 50 40 30 20 10 0 -10 -10
10 µm
(b)
(a)
-20
Compound
80 70 60 50 40 30 20 10 0 -10
Fe Cr O Ga Ni
-20
-10
0
10
20
Depth (µm)
(d)
Fig. 8. SEM images of specimens exposed for (a) 17 h, (b) 700 h, and elemental profiles by EPMA for (c) 17, and (d) 700 h to liquid gallium at 500 °C in vacuum cover-gas condition.
critical value of oxygen concentration beyond which the solubility of a metallic constituent is independent of the oxygen concentration. The air cover-gas condition in this study is equivalent to liquid metal environment with saturated oxygen concentration, but the solubility limit of oxygen (1.02 105 at.%) in liquid gallium at 500 °C is much lower than that of metallic constituents including Fe, Cr and Ni. In addition, the overall corrosion process in this study is mainly controlled by the diffusion of metallic constituents of SS316L as described previously. Therefore, it is believed that the oxygen concentration change did not make a significant difference in the corrosion kinetics of SS316L exposed to liquid gallium at the temperature in this study. Features of fissure in the reaction layers after the longest exposure to liquid metals were observed in all test conditions. This type of morphology has also been reported in the case of niobium oxidation in the results of Valot et al. [29], and they have proposed an interpretation taking account of the Pilling–Bedworth ratio associated with the oxidation process. A higher Pilling–Bedworth ratio than 1 leads to compressive stresses in the oxide. According to Valot et al. [29], the reactivity would be enhanced at the edges, locally explaining the oxide layer thickness and the associated stresses. By analogy with the approach developed in oxidation, Barbier and Blanc [8] assumed that stresses are also involved during the layer formation and provide a driving force for the formation of fissures. In the case of SS316L exposed to liquid gallium in this study, the Pilling–Bedworth ratios of FeGa3/Fe, CrGa4/Cr and Ni2Ga3/Ni are estimated to be 5.4, 7.4 and 6.6, respectively, which are higher than 1. As a consequence, the maximum curvature appears at the edges, which promotes the initiation and the development of fissures. Therefore, it is believed that fissures are related with the enhanced reactivity and evolution of stress at the edges during the formation of reaction layer.
Based on the results of this study, AISI 316L stainless steels showed the reduced metal loss, weight change and reaction layer thickness in gallium alloy environments than in pure gallium at a high temperature. However, the thickness of the reaction layer formed, even in gallium alloy environments, is considered relatively larger than what can be generally expected from other liquid metal coolants, including Na, Pb and Pb–Bi. The formation of a thick layer on the structural materials is not usually favored by the reactor designer, because of possible changes in the thermal and mechanical characteristics, even though investigation into the mechanical properties of the reaction layer is necessary. Therefore, unless the mechanical properties in structural components with reaction layer formations meet the design requirements, the authors would suggest a detailed study to improve the compatibility of structural materials with liquid gallium and gallium alloys. The pathways could be used for proper alloying of the structural materials to form a protective layer on the structural components. This technique has been extensively studied in Pb or Pb–Bi environments by using the formation of a surface layer including Al2O3 and SiO2 [30–32]. The investigation is ongoing, into the effects of a protective surface layer on the material compatibility with liquid gallium. A series of tests with pre-oxidized samples with Al2O3 and SiO2 are in progress, and it is expected that the results will be released in the near future.
5. Summary and conclusion The interaction between austenitic stainless steel, AISI 316L and liquid metals, including gallium and gallium alloys, has been investigated at a high temperature in different cover-gas conditions for the potential application to advanced fast reactor coolants.
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(a)
Weight change (mg/cm 2)
400
400
As-received specimen (Air condition)
200
Weight change (mg/cm 2)
(a)
0 -200 -400 -600
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
-800 0
As-received specimen (Vacuum condition)
200 0 -200 -400
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
-600 -800
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
Exposure time (hr)
(b)
Metal loss squared (mm2 )
0.08 0.07
Metal loss squared (mm 2 )
(b)
Exposure time (hr)
As-received specimen (Air condition)
0.06 0.05 0.04 0.03 0.02
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.01 0.00 0
0.08
As-received specimen (Vacuum condition)
0.07 0.06 0.05 0.04 0.03 0.02
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.01 0.00
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
Exposure time (hr)
(c)
6
1.2x10
6
1.0x10
As-received specimen (Air condition)
5
8.0x10
5
6.0x10
5
4.0x10
5
2.0x10
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.0 0
Reaction layer squared (µm2)
(c)
Reaction layer squared (µm 2 )
Exposure time (hr) 6
1.2x10
6
1.0x10
As-received specimen (Vacuum condition)
5
8.0x10
5
6.0x10
5
4.0x10 2.0x10
5
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.0 0
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
Exposure time (hr)
Exposure time (hr) Fig. 9. Measurement of (a) weight change, (b) metal loss, and (c) reaction layer thickness of specimens exposed to three different liquid metals at 500 °C in air cover-gas condition.
From the results of this study, the following summary can be made. – Austenitic stainless steel, AISI 316L was considerably corroded by all gallium liquid metals, but the weight changes, metal losses and reaction layer thickness of specimens were significantly reduced in Ga–14Sn–6Zn and Ga–8Sn–6Zn comparing to those in pure Ga at a high temperature. It is because the corrosion is mainly controlled by diffusion and the diffusivities of metallic constituents are lower in liquid gallium alloy than in liquid gallium in this study. – By the interaction between AISI 316L and liquid metals, there is a formation of a reaction layer composed of major constituents, including Fe, Cr, and Ni from test specimens, as well as those from liquid metals, including Ga and Zn. Specimens tested in high temperature gallium alloys (Ga–14Sn–6Zn and Ga–8Sn– 6Zn), zinc with 1–2 wt.% and tin with 0.1–0.2 wt.% were detected in the reaction layer near the interface.
Fig. 10. Measurement of (a) weight change, (b) metal loss, and (c) reaction layer thickness of specimens exposed to three different liquid metals at 500 °C in vacuum cover-gas condition.
Table 2 The thermodynamic equilibrium phases at isothermal (500 °C) binary system in pure gallium environment [17–19]. System
Fe–Ga
Cr–Ga
Ni–Ga
Phase
aFe3Ga aFe6Ga5
aCr3Ga4
a0
Cr5Ga6 CrGa4
Ni5Ga3 Ni3Ga2 Ni3Ga4 Ni2Ga3
Fe3Ga4 FeGa3
– Features of fissure were observed in the reaction layers with longer exposure, in all test conditions, and it is believed that the fissures are related to the change in the growth kinetics and the evolution of stress in the reaction layer at the edges. While AISI 316L exhibited the reduced metal loss, weight change and reaction layer thickness in gallium alloy environments than in pure gallium at a high temperature, the thickness of the reaction layer formed is considered still larger than what can be
S.H. Shin et al. / Journal of Nuclear Materials 422 (2012) 92–102
Fig. 11. TOF-SIMS analysis on the surface of reaction layer formed on specimens that exposed to pure gallium for 700 h in air condition.
Fig. 12. TOF-SIMS analysis on the surface of reaction layer formed on specimens that exposed to Ga–14Sn–6Zn for 700 h in air condition.
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Table 3 The estimated diffusion coefficients of metallic constituents in liquid gallium and alloys at 500 °C Specimen constituent
Estimated diffusivity (x10-5 cm2/s)
Fe Cr Ni
In Ga 25.48 26.41 24.86
In Sn 8.205 8.503 11.38
In Zn 3.225 3.748 3.32
generally expected from other liquid metal coolant conditions. The authors would suggest that a detail study to improve the compatibility of structural materials by proper alloying to form a protective layer such as Al2O3 and SiO2 needs to be pursued. Acknowledgments This work was financially supported by the Human Resources Development Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (MKE) and by the Korean Nuclear R&D program organized by the National Research Foundation (NRF) of Korea in support of the Ministry of Education, Science and Technology (MEST). References [1] T. Sawada, A. Netchaev, H. Ninokata, H. Endo, Prog. Nucl. Energy 37 (2000) 313–319. [2] V.Ya. Prokhorenko, V.V. Roshchupkin, M.A. Pokrasin, S.V. Prokhorenko, V.V. Kotov, High Temp. 38 (2000) 954–968.
[3] R.I. Jaffee, J.S. Smith, E.A. Fromm, B.W. Gonser, AECD-3317, ATI No. 163533, Battelle Memorial Institute, Tennessee, US, 1949. [4] P.R. Luebbers, W.F. Michaud, O.K. Chopra, ANL-93/31, Argonne National Laboratory, 1993. [5] IAEA-THPH, Thermophysical Properties of Materials for Nuclear Engineering: a Tutorial and Collection of Data, IAEA, Vienna, 2008. [6] W.D. Wilkinson, ANL Report No. 5027, Argonne National Laboratory, 1953. [7] C.F. Old, J. Nucl. Mater. 92 (1980) 2–25. [8] F. Barbier, J. Blanc, J. Mater. Res. 14 (1999) 737–744. [9] J.D. Harrison, C. Wagner, Acta Metall. 7 (1959) 722–735. [10] W.D. Manly, ORNL Report No. 2055, Oak Ridge National Laboratory, 1956. [11] S.H. Shin, M.S. Thesis, Ulsan National Institute of Science and Technology, 2011. [12] S.P. Yatsenko, N.A. Sabirzyanov, A.S. Yatsenko, J. Phys.: Conf. Ser. 98 (2008) 1–7. [13] M.A. Bright, Ph.D. Thesis, Morgantown, West Virginia University (2007). [14] M.I. Pashechko, Kh.B. Vasyliv, Mater. Sci. 31 (1995) 485–493. [15] Z. Moser, J. Phase Equilib. 13 (1994) 172–176. [16] M. Venkatraman, J.P. Neumann, Bull. Alloy Phase Diagr. 9 (1988) 159–162. [17] H. Okamoto, Bull. Alloy Phase Diagr. 11 (1990) 576–581. [18] H. Okamoto, J. Phase Equilib. Diffus. 28 (2007) 301. [19] W.X. Yuan, Z.Y. Qiao, H. Ipser, G. Eriksson, J. Phase Equilib. Diffus. 25 (2004) 68–74. [20] V. Raghavan, J. Phase Equilib. Diffus. 28 (2007) 399. [21] N.H. Nachtrieb, Adv. Phys. 16 (1967) 309–323. [22] G. Careri, A. Paoletti, Il Nuovo Cimento 1 (1955) 161–165. [23] J. Petit, N.H. Nachtrieb, J. Chem. Phys. 24 (1956) 1056. [24] G. Thomaes, J.V. Itterbeek, Mol. Phys. 2 (1959) 372–378. [25] M.L. Giorgi, P. Durighello, R. Nicolle, J.B. Guillot, J. Mater. Sci. 39 (2004) 5803–5808. [26] T. Ishida, Metall. Trans. B 17B (1986) 281–289. [27] D.E. Langberg, M. Nilmani, Metall. Mater. Trans. B 27B (1996) 780–787. [28] J. Zhang, P. Hosemann, S. Maloy, J. Nucl. Mater. 404 (2010) 82–96. [29] C. Valot, D. Ciosmak, M. Lallemant, Oxid. Met. 41 (1994) 235–250. [30] G. Muller, G. Schumacher, F. Zimmermann, J. Nucl. Mater. 278 (2000) 85–95. [31] J. Lim, H.O. Nam, I.S. Hwang, J.H. Kim, J. Nucl. Mater. 407 (2010) 205–210. [32] M.P. Short, S.E. Ferry, S. Morton, R.G. Ballinger, Int. Heat Treat. Surface Eng. 4 (2010) 74–80.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
A study on corrosion behavior of austenitic stainless steel in liquid metals at high temperature Sang Hun Shin, Jong Jin Kim, Ju Ang Jung, Kyoung Joon Choi, In Cheol Bang, Ji Hyun Kim ⇑ Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), 100 Banyeon-ri, Eonyang-eup, Ulju-gun, Ulsan 689 798, Republic of Korea
a r t i c l e
i n f o
Article history: Received 21 July 2011 Accepted 10 December 2011 Available online 28 December 2011
a b s t r a c t The purpose of this study is to investigate the interaction between austenitic stainless steel, AISI 316L, and gallium liquid metal at a high temperature, for the potential application to advanced fast reactor coolants. Test specimens of AISI 316L were exposed to static gallium at 500 °C for up to 700 h in two different cover-gas conditions, including air and vacuum. Similar experimental tests were conducted in gallium alloy liquid metal environments, including Ga–14Sn–6Zn and Ga–8Sn–6Zn, in order to study the effect of addition of alloying elements. The results have shown that the weight change and metal loss of specimens were generally reduced in Ga–14Sn–6Zn and Ga–8Sn–6Zn compared to those in pure gallium at a high temperature. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction For recycling spent fuels from an operating nuclear power plant, the liquid metal fast breeder reactor (LMFBR) is one of the most promising candidates among Gen-IV nuclear energy systems. Among the various liquid metals used as a primary coolant, sodium is a spotlighted coolant for designing fast breeder reactors. However, sodium has a disadvantage in that it has high activity with water and air. This factor drives the search for alternatives. In this aspect, heavy liquid metals, including pure lead and lead–bismuth eutectic (LBE) alloy, have been extensively studied for fast reactor application. However, there have also been issues such as corrosion with structural alloys at a high temperature, and Po-generation in LBE cooled nuclear systems. This study focused on another liquid metal, gallium, as a potential coolant for the next generation nuclear reactor system. The element of gallium possesses unique properties as follows: It is very stable in air or water, has a very low melting point, and a very high boiling point. It melts at 29 °C and the boiling point is as high as 2204 °C, which gives a designer a larger margin before coolant boiling if severe accidents happen. Being a liquid metal, the heat transfer characteristics would be good, though not as good as some other liquid metals, such as sodium. There is no issue with gallium in terms of chemical activity, or radioactivity increase by irradiation in a nuclear reactor environment. Because of these advantages in using liquid gallium, there have been several studies in the past for applying liquid gallium to nuclear systems [1–3]. The absorption cross section of gallium is rather high at 2.2 barns per atom ⇑ Corresponding author. Tel.: +82 52 217 2913. E-mail address: [email protected] (J.H. Kim). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.12.007
[4], which is a drawback, but it can be managed to a certain degree, because this could be reduced by proper alloying with elements with a lower absorption cross section. When gallium is alloyed with tin and zinc which have a cross section of 0.63 and 1.10 barns per atom, respectively, in a certain ratio, the melting point would be 21 °C. The thermal conductivity of pure gallium and alloying elements with gallium, tin and zinc at 30 °C is 29 W/m K, 66 W/ m K, and 116 W/m K, respectively [4,5]. Tin and zinc are the materials which have both the ternary eutectic and a lower absorption cross section than gallium [3]. When viewed for compatibility as a coolant, which is the main interest of this study, gallium has a relatively high affinity for many metals and alloys, especially steels. Therefore, there has been a relatively high corrosion of these structural metals and alloys, due to the dissolution of their various constituents by the liquid gallium [3,6–8]. Since gallium had some promise as a reactor coolant, owing to its unique properties as described above, the studies on determining the effects of alloying on the melting point and cross section, and on understanding the corrosion of possible structural materials by gallium, were merited. In this study, a series of corrosion tests of structural materials were performed in static gallium at 500 °C for up to 700 h, in both air and vacuum cover-gas conditions, and analyzed to understand the general corrosion behaviors of structural materials in liquid gallium at a high temperature. Also, the effect of adding alloying elements, including tin and zinc, to gallium were investigated. 2. Experimental In this study, austenitic stainless steel, AISI 316L, one of the most popular structural materials in the nuclear industry, was used
S.H. Shin et al. / Journal of Nuclear Materials 422 (2012) 92–102 Table 1 Chemical composition of SS 316L used in this study. Element
Fe
Cr
Ni
Mo
Mn
Si
Other
wt.%
Bal.
16.43
10.05
2.02
1.02
0.66
–
Fig. 1. Optical micrograph of austenitic 316L stainless steel (magnification: 200).
for the various corrosion tests. Table 1 shows the chemical composition of the AISI 316L used in this study. A specimen was etched with 100 mL of ethanol, 100 mL of HCl and 5 g of CuCl2 for 30 s at room temperature. Fig. 1 shows the typical microstructure of the material, obtained by optical microscope after etching. All specimens for the corrosion test had dimensions of 30 mm in length, 10 mm in width and 3 mm in thickness, and were cut from an AISI 316L plate by a high pressure water jet process to avoid thermal stress. Prior to exposure to gallium, each specimen was mechanically polished by SiC papers, diamond suspension (6 lm, 3 lm, and 1 lm), and finally alumina paste down to 0.04 lm, then ultrasonically cleaned with demineralized water for 30 min. Each specimen was placed in an alumina crucible filled with liquid gallium heated to about 40 °C to avoid precipitate from the container. Tests were conducted in two different cover-gas condi-
93
tions, including air and vacuum. After introduction of the specimen, the crucible was placed in a furnace with air for the test in air cover-gas conditions. For the test at high vacuum cover-gas conditions, the crucible was placed in a furnace with a diffusion pump operating up to 5 106 torr. All the specimens were exposed to gallium at 500 ± 5 °C for times between 17 and 700 h. After exposure to gallium and gallium alloys, all specimens were first rinsed in 50 °C water to remove gallium residue from the surface, and then ultrasonically cleaned in demineralized water to ensure complete removal of gallium. After drying, all specimens were weighed, and their dimensions were measured. Subsequently, the specimens were sectioned perpendicular to their length and mechanically polished to examine the cross sections by optical and scanning electron microscopy (SEM) composition of the phases was determined by using energy dispersive X-ray spectrometry (EDS), electron probe X-ray microanalysis (EPMA), and time of flight secondary ion mass spectrometry (TOF-SIMS). Elemental concentration profiles were obtained in the reaction zone to determine the composition of the phases. To quantify the corrosion behavior, the weight change, metal loss, and thickness of the reaction layer were measured in each post-test specimen. For these measurements, both sides of the rectangular cross section were considered.
3. Results Fig. 2 shows the typical optical micrographs of the cross-section around the edge region of post-test specimens. As shown in Fig. 2, specimens exposed to gallium for 17 h still retain their rectangular shape, but those exposed to gallium for 700 h lost the corners of the rectangles. The thickness of the reaction layer formed by the interaction between steel and gallium was not uniform, especially at the corners of the rectangle, but those on the side flat surfaces were observed to be uniform in each specimen, because the reaction layer dominantly formed to width direction with time. Also, with the increase of the exposure time, the volume and weight of the specimens decreased, but the thickness of the reaction layer increased in general. There is no clear evidence for a preferential
Fig. 2. Optical images of specimens after exposure to liquid gallium at 500 °C for (a) 17 h, (b) 700 h in air cover gas condition, (c) 17 h, and (d) 700 h in vacuum cover-gas condition, respectively.
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Fig. 3. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 17 h at 500 °C in air cover-gas condition.
attack along the grain boundary which is commonly observed in some liquid metal environments at a high temperature [4,8–10]. 3.1. Corrosion tests in Ga liquid metal All the specimens were found to form reaction layers on the outer. Figs. 3 and 4 show the SEM images and maps of cross sectional chemical composition around the reaction layers, formed on the specimens exposed to liquid gallium with air cover-gas at 500 °C for 17 and 700 h, respectively. The SEM images (5000 magnification) in Fig. 5 show the clear interface between the base metal and the compound region. Referring to the grain structure, as shown in Fig. 1, it is evident that there is no preferential attack along grain boundaries, along with the quantitative elemental profiles around the interface between the base metals and the reaction layers by the EPMA of the post-test samples. Comparing the SEM images in Figs. 3a and 4a, the thickness (250 lm) of the reaction layer in the specimens exposed to liquid gallium for 17 h is much thinner than that (850 lm) of the specimens exposed to liquid gallium for 700 h. It is also noted that there exists a fissure for longer exposure tests as shown in Fig. 4a. The results of the maps of the cross sectional chemical composition show that the reaction layers are mainly composed of gallium from the liquid metals bath, and iron, chromium, and nickel from the test specimens. The solubility data of the specimen constituents in gallium has not been widely studied. The main constituents of the specimen are iron, chromium and nickel. At
500 °C, the solubility of iron, chromium and nickel in gallium is 0.104 at.%, 0.049 at.% and 1.34 at.%, respectively. Nickel is highly soluble in gallium compared to other constituents at the temperature. Oxygen observed on the outermost surface of the reaction layer is considered to form a thin layer of oxide with gallium in the beginning stage of the reaction. As seen from elemental profiles by EPMA (Fig. 5) along the interface between the base metals and the reaction layers, transition of each chemical element is much sharper in the specimens subjected to longer exposure, than those exposed for shorter lengths of time. Figs. 6–8 show the SEM images, maps of cross sectional chemical composition, and the quantitative EPMA profiles, around the reaction layers formed on the specimens exposed to liquid gallium with vacuum cover-gas at 500 °C for 17 and 700 h, respectively. Compared to the results in air cover-gas conditions, the thickness of the reaction layer in the vacuum cover-gas conditions is slightly reduced (200 lm) after 17 h exposure to gallium. However, after 700 h exposure it showed a significant gain (1000 lm), as seen from Figs. 6a and 7a. The EPMA maps of the cross sectional chemical composition and elemental profiles around the reaction layer did not show a significant difference between the two different cover-gas conditions. 3.2. Corrosion tests in Ga–14Sn–6Zn liquid metal As same as performed in the specimens exposed in pure Ga liquid metal, maps of cross sectional chemical composition, and
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Fig. 4. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 700 h at 500 °C in air cover-gas condition.
the quantitative elemental profiles by EPMA are acquired from the reaction layers formed on the specimens exposed to Ga–14Sn–6Zn liquid metal with air cover-gas at 500 °C for 17 and 700 h, respectively. Detail images and elemental profiles are given in a recent study of Shin [11]. Similar to the results in the pure liquid gallium test, there is no preferential attack along grain boundaries, but a significant change is seen in the thickness of the reaction layer with increased exposure time in Ga–14Sn–6Zn liquid metal. Compared to the thickness of the reaction layer formed in the pure gallium test, a relatively thinner layer is observed to develop. The reaction layers are seen to reach to thicknesses of about 200 lm after 17 h, and 750 lm after 700 h in air cover-gas conditions, while those in vacuum cover-gas conditions are observed to grow to about 200 lm after 17 h and 800 lm after 700 h. In the literature, the solubility of iron in gallium at 500 °C is 0.104 at.%, while iron in zinc is 0.128 at.%, respectively [12–13]. The solubility of iron in tin at 500 °C is not available, but it shows almost same value of that in zinc at 800 °C [14]. In the case of chromium, the solubility limit in gallium, tin, and zinc at 500 °C is 0.049 at.%, 0.096 at.% and 1.48 at.%, respectively [12,15–16]. The solubility of nickel in gallium is 1.34 at.%, but there is little previous study of nickel’s solubility in tin and zinc. In AISI 316L stainless steel which is mainly composed of 69 wt.% of iron and 16.4 wt.% of chromium in this study, therefore, it would be expected that the amount of SS316L dissolution in tin, and probably in zinc, are larger than that in gallium at this
temperature. However, these solubility limits for both iron and chromium in each liquid metal does not agree with the observation in this study. The test results show that all specimens exposed to pure gallium have higher metal loss and larger reaction layer thickness than those exposed to gallium alloys at the temperature. The possible reason for this behavior is that the corrosion process is mainly controlled by diffusion, not by solubility limit, which will be discussed in detail in Section 4. A feature of fissure is also observed in longer exposure tests, irrespective of cover-gas conditions. In the chemical composition around the reaction layer, specimens exposed to Ga–14Sn–6Zn showed a slight difference compared to those exposed to pure gallium. While the reaction layers are mainly composed of elemental species from both the liquid metal bath and the base metal, similar to the specimens exposed to pure gallium liquid metal, the major difference is that small amounts of zinc (1–2 wt.%) and tin (0.1– 0.2 wt.%) were detected in the reaction layer, as seen from the cross sectional chemical composition maps, as well as in the quantitative elemental profiles by EPMA. 3.3. Corrosion tests in Ga–8Sn–6Zn liquid metal As with the results of the tests in Ga–14Sn–6Zn liquid metal, a significant gain in thickness of the reaction layer was observed with increased exposure time in Ga–8Sn–6Zn liquid metal. The reaction layer was also found to contain a small amount of zinc
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Compound
Base metal
Base metal
Compound
10 µm
(b)
80 70 60 50 40 30 20 10 0 -10
Fe Cr O Ga Ni
wt (%)
wt (%)
(a) 80 70 60 50 40 30 20 10
Fe Cr O Ga Ni
0 -10 -20
-10
0
10
20
Depth (µm)
(c)
-20
-10
0
10
20
Depth (um)
(d)
Fig. 5. SEM images of specimens exposed for (a) 17 h, (b) 700 h, and elemental profiles by EPMA for (c) 17, and (d) 700 h to liquid gallium at 500 °C in air cover-gas condition. Red (base metal) and blue (compound region) spots in SEM images (above) indicate the analyzed positions for EPMA, and quantitative elemental profiles of Fe, Cr, O, Ga, and Ni are obtained by EPMA (below). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(1–2 wt.%) from the quantitative elemental profiles by the EPMA measurements. In this case, there is no preferential attack along grain boundaries, as same as observed in pure gallium and Ga– 14Sn–6Zn. 3.4. Measurement of weight change, metal loss and reaction layer thickness The weight change, metal loss, and reaction layer of specimens after exposure to gallium and gallium alloys at high temperature were measured with post-test specimens, and the summarized results as a function of exposure time are plotted in Fig. 9 for air cover-gas tests, and in Fig. 10 for vacuum cover-gas tests, respectively. Generally, the weight of specimens decreased, but metal loss and the thickness of the reaction layer increased with the increase of the exposure time. In the air cover-gas conditions, the weight of the specimens was observed to change in negative proportion to the exposure time in liquid pure gallium, but the liquid gallium alloys showed a different pattern of change; a parabolic decrease with increase of time. After 700 h exposure, the weight change (804 mg/cm2) of specimens in pure gallium is about 2–3 times higher than that in Ga–14Sn–6Zn (459 mg/cm2) or Ga–8Sn–6Zn (324 mg/cm2) as shown in Fig. 9a. Also, the metal loss and reaction layer thickness increase with exposure time, which is commonly observed in the oxidation of metals and alloys at high temperatures. As shown in Fig. 9b, c and Fig. 10b, c, even during the shorter exposure time (17 h), the significant changes in both metal loss and reaction layer thickness were observed, which affects the weight change consequently as
they are directly related. However, the time dependence of the data squared shows that one obtains straight lines only after a transient period of about 17 h in most cases. In other words, the variation of the metal loss squared and reaction layer thickness squared versus exposure time cannot be described by a straight line for durations less than 17 h. This indicates that different kinetic regimes are involved during the process. In the initial stage (time less than 17 h), a faster growth appears with nonparabolic kinetics. After this transient period, the plot of the variation of the metal loss and layer thickness squared versus time can be represented by a straight line, indicating a parabolic behavior for the process. The change from nonparabolic to parabolic behavior can also be distinguished by the morphological change of the reaction layer. In the early stage, the reaction layer started to grow forming a cruciform pattern with sharp corners as shown in Fig. 2a and c whereas rounded edges appeared for longer times as shown in Fig. 2b and d. This type of transitional kinetic behavior of 316L steel has been reported for SS316L exposed to pure gallium with vacuum cover-gas condition in the previous study [8]. In the current study, the similar kinetics change for the weight loss and reaction layer thickness are observed in SS316L specimens exposed to gallium alloys with different cover-gas conditions. Compared to the data in gallium alloy conditions, specimens tested in pure gallium showed larger metal loss and layer thickness in all exposure times, except in metal loss after 700 h in Ga–8Sn– 6Zn. The wetted residue of liquid metal in post-test specimens could cause the scattering in measurement. As shown in Fig. 10a, in vacuum cover-gas conditions, the result showed that the weight change after 700 h exposure is 659 mg/cm2 in pure gallium, but 239 mg/ cm2 and 332 mg/cm2 in Ga–14Sn–6Zn and Ga–8Sn–6Zn,
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Fig. 6. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 17 h at 500 °C in vacuum covergas condition.
respectively. Compared to the results tested in air cover gas conditions, specimens tested in vacuum cover-gas conditions showed less weight change in all liquid metal conditions. The metal loss and layer thickness showed the parabolic increase with exposure time, the same as those in air cover-gas conditions. Specimens tested in pure gallium conditions showed larger metal loss and layer thickness in all exposure time than those tested in gallium alloy conditions, as shown in Fig. 10b and c. There is no clear trend in the relationship between metal loss and layer thickness, and the cover-gas conditions for this test, when comparing the data in the air and vacuum cover-gas conditions.
4. Discussion From the characterization of post-test specimens by SEM, EDS and EPMA in this study, the interaction between austenitic stainless steel, AISI 316L, and liquid metals including gallium and gallium alloys at a high temperature resulted in the formation of a reaction layer consisting of the major constituents of the test specimens including iron, chromium, and nickel, as well as liquid metals including gallium, tin and zinc. From the literature data [17–19], thermodynamic equilibrium phases at isothermal (500 °C) binary system including Fe–Ga, Cr– Ga, Ni–Ga are identified in case of pure gallium environment,
and the summary is given in Table 2. The results of EPMA quantitative analysis on the reaction layer as shown in Section 3 indicates the observance of 65–75 wt.% of gallium, 20–25 wt.% of iron, 3–5 wt.% of chromium and 1–3 wt.% of nickel. These compositions allow gallium rich phases, including FeGa3, CrGa4 and Ni2Ga3 at 500 °C in the reaction layer, and the thermodynamic data [17–19] prove that these phases are likely to be formed due to the lowest formation of energy. TOF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) analysis was also performed in this study and the results indicated the presence of FeGa3, CrGa4 and Ni2Ga3 phases in the reaction layer of the specimen exposed to pure gallium, as shown in Fig. 11. In liquid gallium alloys (Ga–14Sn–6Zn and Ga–8Sn–6Zn), tin and zinc are composed in the liquid bath with a total amount of 20 wt.% and 14 wt.%, respectively. Specimens exposed to these environments also develop reaction layers with different compositions compared to specimens exposed to pure gallium. The authors expected that the reaction layer would be composed of a similar amount of tin and zinc. In liquid gallium–tin–zinc alloy tests, however, small amounts of zinc (1–2 wt.%) and tin (0.1–0.2 wt.%) were detected in the reaction layers by EPMA. TOF-SIMS analysis was also conducted to carefully investigate the phases on the reaction layer. All detected binary alloys in the reaction layer formed in gallium alloy environments, are FeGa3, Ni2Ga3, CrGa4, Sn, Zn, FeSn, FeZn10 and FeZn13 as shown in Fig. 12. From Fig. 12, the strong
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Fig. 7. SEM image and maps of cross sectional chemical composition in the reaction layer formed on specimens exposed to liquid gallium for 700 h at 500 °C in vacuum covergas condition.
peaks of Zn, Sn and FeSn are observed, but the signals of FeZn10 and FeZn13 are weakly detected. Thermodynamically, FeSn and Fe3Zn10 are the most stable phases in the range of chemical composition of iron (70%), tin (8–14%), and zinc (6%) in the range of the test temperature in this study [20]. TOF-SIMS analysis shows the presence of FeSn phase, but not the Fe3Zn10 phase. By combining the results described above, it is considered that the EPMA signals of zinc, not tin, in the reaction layer, comes from the Fe3Zn10 phase formed during the exposure to Ga–Sn–Zn alloy liquid metals. As summarized in the Section 3, the test results showed that all specimens exposed to pure gallium have higher metal loss and larger reaction layer thickness than those exposed to gallium alloys. The solubility limits of metallic constituents in liquid gallium and alloys, by themselves, could not explain the observed corrosion behavior in gallium alloy environments in this study, because the solubilities of iron and chromium in tin, and probably in zinc, are higher than those in gallium at the temperature as described previously. The possible reason for this behavior is that the corrosion process in this study is mainly controlled by diffusion after a nonparabolic stage for short exposure times. Using the self-diffusion coefficients in pure liquid metals given in the literature [21–23], the diffusivities of the metallic constituents can be estimated by the expression of Thomaes and Itterbeek [24]. For the estimation of the diffusivities of the metallic constituents, 22.81, 5,82 and 2.98 are used as the self-diffusion coefficients (DAA, 105 cm2/s) for gallium, tin and zinc at 500 °C, respectively
[21–23], and the estimated results are summarized in Table 3. The comparison with literature data of diffusion coefficient of iron [25] and nickel [26,27] in zinc bath shows that the estimation is well in agreement having the same order of magnitude at 500 °C. From the estimated diffusivities of the metallic constituents in liquid gallium and alloys, it is seen that all the diffusivities of metallic constituents are higher in liquid gallium than in liquid tin and/ or zinc when dissolved metallic constituents diffuse out into the liquid bath. Therefore, it is generally expected that the diffusivities of metallic constituents are higher in liquid gallium than in liquid gallium alloys, and it consequently results in higher metal loss and larger reaction layer thickness in liquid gallium than those in liquid gallium alloys as observed in this study. The results in this study also indicate that there is no clear trend in the relationship between the metal loss and reaction layer thickness, and the cover-gas conditions, when comparing the data in the air and vacuum cover-gas conditions. In the air cover-gas environment, oxygen could be the key impurity that may affect the liquid metal corrosion. Commonly, for a light liquid metal (sodium and sodium–potassium alloy), the corrosion of iron based steels increases with the oxygen concentration in the liquid metal, while for a heavy liquid metal (lead and lead–bismuth alloy), the presence of oxygen may result in protective oxide layers that inhibit the corrosion [28]. According to Zhang et al. [28], the solubility of metallic constituent in liquid metal can increase with the dissolved oxygen concentration, and there exists an upper and lower
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Compound
Base metal
Base metal
Fe Cr O Ga Ni
0
10
20
Depth (µm)
(c)
wt.(%)
wt.(%)
80 70 60 50 40 30 20 10 0 -10 -10
10 µm
(b)
(a)
-20
Compound
80 70 60 50 40 30 20 10 0 -10
Fe Cr O Ga Ni
-20
-10
0
10
20
Depth (µm)
(d)
Fig. 8. SEM images of specimens exposed for (a) 17 h, (b) 700 h, and elemental profiles by EPMA for (c) 17, and (d) 700 h to liquid gallium at 500 °C in vacuum cover-gas condition.
critical value of oxygen concentration beyond which the solubility of a metallic constituent is independent of the oxygen concentration. The air cover-gas condition in this study is equivalent to liquid metal environment with saturated oxygen concentration, but the solubility limit of oxygen (1.02 105 at.%) in liquid gallium at 500 °C is much lower than that of metallic constituents including Fe, Cr and Ni. In addition, the overall corrosion process in this study is mainly controlled by the diffusion of metallic constituents of SS316L as described previously. Therefore, it is believed that the oxygen concentration change did not make a significant difference in the corrosion kinetics of SS316L exposed to liquid gallium at the temperature in this study. Features of fissure in the reaction layers after the longest exposure to liquid metals were observed in all test conditions. This type of morphology has also been reported in the case of niobium oxidation in the results of Valot et al. [29], and they have proposed an interpretation taking account of the Pilling–Bedworth ratio associated with the oxidation process. A higher Pilling–Bedworth ratio than 1 leads to compressive stresses in the oxide. According to Valot et al. [29], the reactivity would be enhanced at the edges, locally explaining the oxide layer thickness and the associated stresses. By analogy with the approach developed in oxidation, Barbier and Blanc [8] assumed that stresses are also involved during the layer formation and provide a driving force for the formation of fissures. In the case of SS316L exposed to liquid gallium in this study, the Pilling–Bedworth ratios of FeGa3/Fe, CrGa4/Cr and Ni2Ga3/Ni are estimated to be 5.4, 7.4 and 6.6, respectively, which are higher than 1. As a consequence, the maximum curvature appears at the edges, which promotes the initiation and the development of fissures. Therefore, it is believed that fissures are related with the enhanced reactivity and evolution of stress at the edges during the formation of reaction layer.
Based on the results of this study, AISI 316L stainless steels showed the reduced metal loss, weight change and reaction layer thickness in gallium alloy environments than in pure gallium at a high temperature. However, the thickness of the reaction layer formed, even in gallium alloy environments, is considered relatively larger than what can be generally expected from other liquid metal coolants, including Na, Pb and Pb–Bi. The formation of a thick layer on the structural materials is not usually favored by the reactor designer, because of possible changes in the thermal and mechanical characteristics, even though investigation into the mechanical properties of the reaction layer is necessary. Therefore, unless the mechanical properties in structural components with reaction layer formations meet the design requirements, the authors would suggest a detailed study to improve the compatibility of structural materials with liquid gallium and gallium alloys. The pathways could be used for proper alloying of the structural materials to form a protective layer on the structural components. This technique has been extensively studied in Pb or Pb–Bi environments by using the formation of a surface layer including Al2O3 and SiO2 [30–32]. The investigation is ongoing, into the effects of a protective surface layer on the material compatibility with liquid gallium. A series of tests with pre-oxidized samples with Al2O3 and SiO2 are in progress, and it is expected that the results will be released in the near future.
5. Summary and conclusion The interaction between austenitic stainless steel, AISI 316L and liquid metals, including gallium and gallium alloys, has been investigated at a high temperature in different cover-gas conditions for the potential application to advanced fast reactor coolants.
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(a)
Weight change (mg/cm 2)
400
400
As-received specimen (Air condition)
200
Weight change (mg/cm 2)
(a)
0 -200 -400 -600
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
-800 0
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In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
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0
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Exposure time (hr)
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0.08 0.07
Metal loss squared (mm 2 )
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As-received specimen (Air condition)
0.06 0.05 0.04 0.03 0.02
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.01 0.00 0
0.08
As-received specimen (Vacuum condition)
0.07 0.06 0.05 0.04 0.03 0.02
In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.01 0.00
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
Exposure time (hr)
(c)
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1.2x10
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As-received specimen (Air condition)
5
8.0x10
5
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4.0x10
5
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In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.0 0
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(c)
Reaction layer squared (µm 2 )
Exposure time (hr) 6
1.2x10
6
1.0x10
As-received specimen (Vacuum condition)
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8.0x10
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6.0x10
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In pure gallium In Ga-14Sn-6Zn In Ga-8Sn-6Zn
0.0 0
100 200 300 400 500 600 700 800
100 200 300 400 500 600 700 800
Exposure time (hr)
Exposure time (hr) Fig. 9. Measurement of (a) weight change, (b) metal loss, and (c) reaction layer thickness of specimens exposed to three different liquid metals at 500 °C in air cover-gas condition.
From the results of this study, the following summary can be made. – Austenitic stainless steel, AISI 316L was considerably corroded by all gallium liquid metals, but the weight changes, metal losses and reaction layer thickness of specimens were significantly reduced in Ga–14Sn–6Zn and Ga–8Sn–6Zn comparing to those in pure Ga at a high temperature. It is because the corrosion is mainly controlled by diffusion and the diffusivities of metallic constituents are lower in liquid gallium alloy than in liquid gallium in this study. – By the interaction between AISI 316L and liquid metals, there is a formation of a reaction layer composed of major constituents, including Fe, Cr, and Ni from test specimens, as well as those from liquid metals, including Ga and Zn. Specimens tested in high temperature gallium alloys (Ga–14Sn–6Zn and Ga–8Sn– 6Zn), zinc with 1–2 wt.% and tin with 0.1–0.2 wt.% were detected in the reaction layer near the interface.
Fig. 10. Measurement of (a) weight change, (b) metal loss, and (c) reaction layer thickness of specimens exposed to three different liquid metals at 500 °C in vacuum cover-gas condition.
Table 2 The thermodynamic equilibrium phases at isothermal (500 °C) binary system in pure gallium environment [17–19]. System
Fe–Ga
Cr–Ga
Ni–Ga
Phase
aFe3Ga aFe6Ga5
aCr3Ga4
a0
Cr5Ga6 CrGa4
Ni5Ga3 Ni3Ga2 Ni3Ga4 Ni2Ga3
Fe3Ga4 FeGa3
– Features of fissure were observed in the reaction layers with longer exposure, in all test conditions, and it is believed that the fissures are related to the change in the growth kinetics and the evolution of stress in the reaction layer at the edges. While AISI 316L exhibited the reduced metal loss, weight change and reaction layer thickness in gallium alloy environments than in pure gallium at a high temperature, the thickness of the reaction layer formed is considered still larger than what can be
S.H. Shin et al. / Journal of Nuclear Materials 422 (2012) 92–102
Fig. 11. TOF-SIMS analysis on the surface of reaction layer formed on specimens that exposed to pure gallium for 700 h in air condition.
Fig. 12. TOF-SIMS analysis on the surface of reaction layer formed on specimens that exposed to Ga–14Sn–6Zn for 700 h in air condition.
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Table 3 The estimated diffusion coefficients of metallic constituents in liquid gallium and alloys at 500 °C Specimen constituent
Estimated diffusivity (x10-5 cm2/s)
Fe Cr Ni
In Ga 25.48 26.41 24.86
In Sn 8.205 8.503 11.38
In Zn 3.225 3.748 3.32
generally expected from other liquid metal coolant conditions. The authors would suggest that a detail study to improve the compatibility of structural materials by proper alloying to form a protective layer such as Al2O3 and SiO2 needs to be pursued. Acknowledgments This work was financially supported by the Human Resources Development Program of Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (MKE) and by the Korean Nuclear R&D program organized by the National Research Foundation (NRF) of Korea in support of the Ministry of Education, Science and Technology (MEST). References [1] T. Sawada, A. Netchaev, H. Ninokata, H. Endo, Prog. Nucl. Energy 37 (2000) 313–319. [2] V.Ya. Prokhorenko, V.V. Roshchupkin, M.A. Pokrasin, S.V. Prokhorenko, V.V. Kotov, High Temp. 38 (2000) 954–968.
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